A typical electrical discharge machining apparatus comprises means for holding a workpiece constituting a first electrode, a tool electrode spacedly juxtaposed with the first electrode across a dielectric filled machining gap, means for applying discharge-voltage pulses across the two electrodes to effect breakdown of the gap and produce an electrical discharge of a transient or short duration so that workpiece material is eroded or electrically removed, a servosystem or the like for maintaining the gap width at an optimum level as the material removal progresses, and means for supplying a dielectric liquid to the gap to sweep the removed detritus and discharge products therefrom.
Typical electrical discharge machining operations include rough-machining in which a high removal rate is obtained with a relatively rough machined surface, finish-machining in which an increased surface smoothness is attained with a reduced removal rate, and "medium" operations between these two extremes. For rough machining purposes, it is now possible to establish a "no wear" or low wear condition which permits the relative wear of the tool electrode to the workpiece material to be held at 0.1 to 1 percent. In order to establish these distinct machining operating conditions, parameters of the machining discharge are set at suitable levels taking into account the nature of the tool electrode and the workpiece material and the nature of liquid dielectric.
Even where "no wear" operation is desired, it has been necessary to permit a certain degree of tool wear in order to achieve a practically acceptable removal rate or to have the time expended for a particular machining purpose be practically acceptable.
It has been observed that the wear, under existing systems, tends to occur predominantly at contoured corners, ridge or tip portions of the tool electrode and, if the latter has a sharp cavity or recessed portion, that area of the workpiece which is directly juxtaposed thereto tends to be cut excessively compared with the other workpiece region. This has rendered it difficult to obtain a desired machining accuracy and the use of a number of electrodes for a single machining operation or limited the life of the tool at an undesired level.